Transistor matching test fixture

I had the need to match a batch of tran­sis­tor pairs for a project, so I designed a cir­cuit board based on a cir­cuit by Ian Fritz.

Transistor match­ing cir­cuit boards

I had a lit­tle over one hun­dred tran­sis­tors to sort thru, so I built up this small board just to match them. 

Dual Transistor tester schematic
Dual Transistor tester board lay­out top and bottom

The hard­est part of this project was order­ing the matched resis­tors used in the emit­ter to neg­a­tive sup­ply cir­cuit. I thought that I had found the exact resis­tor net­work that I need­ed, but Digikey had an incor­rect descrip­tion of the part. It end­ed up being a 10K x 100K pair which is a total fail for this cir­cuit. Be sure to check the part num­ber against the man­u­fac­tures spec­i­fi­ca­tion sheet to ver­i­fy you are order­ing the part you need, instead of trust­ing the sell­ers part descrip­tion.
What I final­ly end­ed up with was a Vishay MPMT2003AT1 resis­tor array with a ratio tol­er­ance of ± 0.05 % and a ratio drift of ± 2 ppm/°C. It was sup­plied in a SOT-23 pack­age, and need­ed to be sol­dered to a pin head­er to match the board socket.

SOT-23 resis­tor net­work sol­dered to head­er and SIP network

With 100K Ohm resis­tors the cur­rent pass­ing thru the emit­ter is around 0.126 mA, and by con­nect­ing a volt­meter to each of the emit­ters we can mea­sure the dif­fer­en­tial emit­ter volt­age of the two tran­sis­tors.
Different val­ues of matched emit­ter resis­tor pairs can be used to change the cur­rent pass­ing thru the tran­sis­tor, such as a 10K Ohm pair for around 1.2 mA of cur­rent. For fine adjust­ment the pow­er sup­plies can be adjust­ed.
To see if there is any mea­sure­ment mis­match caused by the matched resis­tors, the matched resis­tor pair mod­ule can be removed and rotat­ed 180 degrees, while leav­ing the tran­sis­tors in the same posi­tion, and then com­pare the mea­sure­ments from both positions.

Testing a pair of NPN tran­sis­tors using ± 12 volt sup­plies and multimeter

My plans are to mount the board in a small enclo­sure to pre­vent air cur­rents from affect­ing the mea­sure­ments, but still have easy access for chang­ing out the transistors.

EagleCAD V7.7 Transistor tester dual rev. 1.0 schemat­ic and board ZIP file

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